System and method for controlling the position of a levitated rotor

ABSTRACT

A rotary machine is provided which may include a rotor and a stator within a housing. The stator may be for generating a rotating magnetic field for applying a torque to the rotor. A commutator circuit may provide a plurality of phase voltages to the stator, and a controller may adjust the plurality of phase voltages provided by the commutator circuit to modify an attractive force of the stator on the rotor to move the rotor in an axial direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/042,431, filed Feb. 12, 2016, and entitled “SYSTEM AND METHOD FORCONTROLLING THE POSITION OF A LEVITATED ROTOR,” the entire disclosure ofwhich is hereby incorporated by reference, for all purposes, as if fullyset forth herein.

U.S. patent application Ser. No. 15/042,431 claims priority to and thebenefit of U.S. Provisional Application No. 62/115,603, filed Feb. 12,2015, and entitled “SYSTEM AND METHOD FOR CONTROLLING THE POSITION OF ALEVITATED ROTOR,” the entire disclosure of which is hereby incorporatedby reference, for all purposes, as if fully set forth herein.

U.S. patent application Ser. No. 15/042,431 claims priority to and thebenefit of U.S. Provisional Application No. 62/115,324, filed Feb. 12,2015, and entitled “SYSTEM AND METHOD FOR CONTROLLING THE POSITION OF ALEVITATED ROTOR,” the entire disclosure of which is hereby incorporatedby reference, for all purposes, as if fully set forth herein.

BACKGROUND OF THE INVENTION

The present invention relates in general to rotary devices, and morespecifically, to improved pumping devices and methods for their control.

One exemplary type of rotary machine is a centrifugal pumping device ormechanical circulatory assist device for treating patients with heartfailure. Many types of circulatory assist devices are available foreither short term or long term support for patients havingcardiovascular disease. For example, a heart pump system known as a leftventricular assist device (LVAD) can provide long term patient supportwith an implantable pump associated with an externally-worn pump controlunit and batteries. The LVAD improves circulation throughout the body byassisting the left side of the heart in pumping blood. Examples of LVADsystems are the DuraHeart® LVAS system made by Terumo Heart, Inc. of AnnArbor, Mich. and the HeartMate II™ and HeartMate III™ systems made byThoratec Corporation of Pleasanton, Calif. These systems typicallyemploy a centrifugal pump with a magnetically levitated impeller to pumpblood from the left ventricle to the aorta. The impeller is formed asthe rotor of the electric motor and rotated by the rotating magneticfield from a multiphase stator such as a brushless DC motor (BLDC). Theimpeller is rotated to provide sufficient blood flow through the pump tothe patient's systemic circulation.

Early LVAD systems utilized mechanical bearings such as ball-and-cupbearings. More recent LVADs employ non-contact bearings which levitatethe impeller using hydrodynamic and/or magnetic forces. In one example,the impeller is levitated by the combination of hydrodynamic and passivemagnetic forces.

There is a trend for making centrifugal blood pumps used as themechanical circulatory support devices more miniaturized to treat abroader patient population, more reliable, and with improved outcomes.To follow this trend, contactless impeller suspension technology hasbeen developed in several pump designs. The principle of this technologyis to levitate the pump impeller using one or a combination of forcesfrom electromagnets, hydrodynamics, and permanent magnets. In themeanwhile, the pump should be hemocompatible to minimize the blood celldamage and blood clot formation. To that end, the bearing gap betweenthe levitated impeller and the pump housing becomes an important factor.A small gap may lead to the high probability of the thrombus formationin the bearing or to elevated hemolysis due to excessive shear stress.Likewise, a large gap can compromise the hydrodynamic bearingperformance and the pump efficiency.

One pump design utilizing active magnetic bearings achieves the desiredbearing gap by levitating the impeller using magnetic fields generatedby electromagnetic coils. However, in such a design there is the needfor a separate bearing control system that includes the position sensorsand electromagnetic coils to control the impeller position. Another pumpdesign levitates the impeller using hydrodynamic thrust bearingscombined with passive magnetic bearings. However, such a design usuallyrequires a small bearing gap to provide sufficient hydrodynamic bearingstiffness to maintain impeller levitation and prevent contacts betweenimpeller and the pump housing. Such a small gap may result in aninsufficient washout and vulnerability to blood clotting thuscompromising hemocompatibility.

Therefore, a solution is needed to enhance the bearing gap to achieveadequate washout without increasing the complexity of the pumpmechanical design and reducing the pump efficiency.

There is a need for a pump that includes an integrated control methodfor controlling the impeller position to enhance the bearing gap withoutincreasing the complexity of the pump mechanical design and reducing thepump efficiency.

There is the need for a blood pump designed to maintain a centeredposition of the impeller to limit hemolysis and thrombosis withoutneeding active control of the stationary levitating magnetic field.

There is a need for pumps which overcomes the above and otherdisadvantages of known designs.

BRIEF SUMMARY OF THE INVENTION

In summary, various aspects of the present invention are directed to arotary machine including a rotor within a housing and having a rotormagnetic structure; a stator on a side of the housing for generating arotating magnetic field for applying a torque to the rotor magneticstructure; a commutator circuit for providing a plurality of phasevoltages to the stator; and a controller for rotating the rotor usingthe commutator circuit and a vector control algorithm. In one embodimentthe controller is configured to adjust the phase voltages to modify anattractive force of the stator on the rotor magnetic structure totranslate the rotor.

In various embodiments, the machine further includes a sensing circuitfor determining a position of the rotor. The controller may beconfigured to calculate successive commanded values for the phasevoltages in response to determined phase currents from the sensingcircuit and a variable commutation angle. The angle for calculating thecommanded values may be determined in response to a phase currentcharacteristic and a rotational speed of the rotor.

In various embodiments, the rotor is levitated by a substantiallyconstant passive magnetic field.

In various embodiments, the controller is configured to move the rotorfrom a first balanced position to a second balanced position.

Various aspects of the invention are directed to a rotary machineincluding a rotor within a housing and having a rotor magneticstructure; a bearing mechanism for suspending the rotor in the housingin a balanced, non-contact manner; motor coils on a side of the housingfor generating a magnetic field to apply a torque on the rotor magneticstructure; at least a first sensing circuit for determining a rotationaland axial position of the rotor; a controller for rotating the rotorusing the motor coils; and an impeller position control mechanism foradjusting a position of the impeller in the housing.

In various embodiments, the bearing mechanism comprises one of ahydrodynamic bearing, magnetic bearing, or combination of the same. Invarious embodiments, the modification of the attractive force on therotor magnetic structure by the controller causes the rotor to move froma first balanced position to a second balanced position. The sensingcircuit may include a plurality of position sensors for detecting theaxial and rotational position of the rotor. The plurality of positionsensors may include Hall-effect sensors. The plurality of positionsensors may include optical sensors. In various embodiments, the rotormagnetic structure includes a plurality of magnetic members.

In various embodiments, the rotary machine is a pump. In variousembodiments, the rotary machine is a blood pump. In various embodiments,the rotor is formed as an impeller.

Various aspects of the invention are directed to a method of operatingthe rotary machine described in any of the paragraphs above.

Various aspects of the invention are directed to a method of operating acentrifugal pump including a stator having windings and an impellerrotating in a non-contact manner within a pump housing, the impellerincluding a magnetic structure, the method includes applying a firstlevitating force on the impeller during rotation; and usingelectromagnetic windings, controlling the position of the impeller inthe pump housing axially and rotationally.

In various embodiments, the first levitating force comprises a passivemagnetic attractive force. The at least second levitating force mayinclude an active magnetic force created by electromagnetic coils. Thecoils may be driven by vector control.

Various aspects of the invention are directed to a method of operating acentrifugal pump including a stator having windings and an impellerrotating in a non-contact manner within a pump housing, the impellerincluding a magnetic structure, the method including applying a firstlevitating force on the impeller during rotation; and usingelectromagnetic windings, controlling the position of the impeller inthe pump housing axially and rotationally.

In various embodiments, the stator windings form the electromagneticwindings for controlling the impeller position. In various embodiments,the rotation of the impeller is controlled by interaction between themagnetic structure in the impeller and AC currents in the motor statorwindings. In various embodiments, the axial position of the impeller iscontrolled by interaction between the magnetic structure in the impellerand DC currents in the motor stator windings. In various embodiments,the method includes using the electromagnetic windings to move theimpeller axially from a first predetermined position to a secondpredetermined within the pump housing.

Various aspects of the invention are directed to a system, method, orcomputer-program product as described herein and/or shown in any of thedrawings.

The systems and methods of the present invention have other features andadvantages which will be apparent from or are set forth in more detailin the accompanying drawings, which are incorporated in and form a partof this specification, and the following Detailed Description of theInvention, which together serve to explain the principles of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an implantable pump as one example of a rotarymachine employing the present invention.

FIG. 2 is an exploded, perspective view of the exemplary centrifugalpump of FIG. 1.

FIG. 3 is a cross-sectional view of the exemplary pump of FIG. 1,illustrating the impeller levitated at a first balanced positiongenerally centered within the pumping chamber in accordance with aspectsof the invention.

FIG. 4 is a schematic view of the exemplary pump of FIG. 1, illustratingthe impeller levitated eccentrically in the pump chamber by the mainbearing components.

FIG. 5 is a block diagram of a pump control system in accordance withthe invention.

FIG. 6 is a line chart depicting the method of controlling the impellerposition in accordance with the invention.

FIG. 7a is a schematic view of the pump of FIG. 1, illustrating theimpeller moved to another eccentric position at the bottom of the pumpchamber.

FIG. 7b is a schematic view of the pump of FIG. 1, illustrating theimpeller moved to yet another eccentric position at the top of the pumpchamber.

FIG. 8 is a flowchart showing a method of controlling impeller positionin accordance with the invention.

FIG. 9a is a flowchart showing a method of controlling impeller positionin accordance with the invention.

FIG. 9b is a line chart depicting a method for moving the impellerbetween two balanced positions in accordance with aspects of theinvention.

FIG. 10 is a flowchart showing a method of controlling impeller positionduring start-up of the pump in accordance with the invention.

FIG. 11 is a cross-sectional view of an exemplary centrifugal flow pumpin accordance with aspects of the invention, illustratingelectromagnetic bearings to supplement the stator assembly positioningcontrol.

FIG. 12 is a cross-sectional view of an axial flow pump in accordancewith aspects of the invention, the axial flow pump including mechanicalbearings.

FIG. 13 is a perspective view of the impeller of FIG. 12, with arrowsdepicting the direction of translation in accordance with the invention.

FIG. 14 is a cross-sectional view of another axial flow pump inaccordance with aspects of the invention, the axial flow pump includingmechanical bearings.

FIG. 15 is a cross-sectional view of another axial flow pump inaccordance with aspects of the invention, the axial flow pump includingpassive magnetic and hydrodynamic bearings.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to those embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the appended claims.

For convenience in explanation and accurate definition in the appendedclaims, the terms “up” or “upper”, “down” or “lower”, “inside” and“outside” are used to describe features of the present invention withreference to the positions of such features as displayed in the figures.

In many respects the modifications of the various figures resemble thoseof preceding modifications and the same reference numerals followed bysubscripts “a”, “b”, “c”, and “d” designate corresponding parts.

As used herein, “gap” generally refers to the secondary flow gaps aroundthe impeller as would be understood by one of skill in the art. Theprimary flow is through the impeller blade regions. The secondary flowgaps are the other areas of fluid, generally around the impeller. Insome respects, the secondary flow gaps are between the impeller and thehousing wall and define the hydrodynamic bearing.

The term “machine-readable medium” includes, but is not limited toportable or fixed storage devices, optical storage devices, wirelesschannels and various other mediums capable of storing, containing orcarrying instructions and/or data. A code segment or machine-executableinstructions may represent a procedure, a function, a subprogram, aprogram, a routine, a subroutine, a module, a software package, a class,or any combination of instructions, data structures, or programstatements. A code segment may be coupled to another code segment or ahardware circuit by passing and/or receiving information, data,arguments, parameters, or memory contents. Information, arguments,parameters, data, etc. may be passed, forwarded, or transmitted via anysuitable means including memory sharing, message passing, token passing,network transmission, etc.

Furthermore, embodiments of the invention may be implemented, at leastin part, either manually or automatically. Manual or automaticimplementations may be executed, or at least assisted, through the useof machines, hardware, software, firmware, middleware, microcode,hardware description languages, or any combination thereof. Whenimplemented in software, firmware, middleware or microcode, the programcode or code segments to perform the necessary tasks may be stored in amachine readable medium. One or more processors may perform thenecessary tasks.

Although aspects of the invention will be described with reference to ablood pump, one will appreciate that the invention can be applied tovarious rotary machines and other types of pumps. The mechanisms andmethods of the invention will be described in relation to blood pumpsand in particular the ability to adjust the impeller operating positionto address performance, such as the attendant risks for thrombus andhemolysis when pumping blood. One will appreciate from the descriptionherein that the invention can be applied broadly to other pumps, rotarymachines, and induction motors.

Aspects of the invention enable to the ability to enhance or control thebearing gap. One might wish to increase the bearing gap to adjust thewashout rate, lubricate the bearing surfaces, or remove materials(particulates, thrombus, etc.) from the bearing gap. Another use of theinvention may be to increase pump efficiency. As is known in the art,the motor efficiency increases as the impeller magnet moves closer tothe motor drive coils. Another use of the invention may be to correctimpeller malpositioning due to bulk forces or external forces (e.g.bumps or movements of the patient's body). These and other advantagescan be achieved without the need for complex control systems inaccordance with the invention.

In one embodiment, an electromagnetic force control method is used tochange the impeller position and enhance the effective gap between theimpeller and the blood chamber. The technique uses the same pump motorstator coils adjust the impeller position as is used to apply a torqueto the impeller. No additional control subsystems and components arenecessary.

Turning now to the drawings, wherein like components are designated bylike reference numerals throughout the various figures, attention isdirected to FIG. 1 which depicts an exemplary pump implanted in a heartfailure patient.

A typical cardiac assist system includes a pumping unit, driveelectronics, microprocessor control unit, and an energy source such asrechargeable batteries and/or an AC power conditioning circuit. Thesystem is implanted during a surgical procedure in which a centrifugalpump is placed in the patient's chest. An inflow conduit is pierced intothe left ventricle to supply blood to the pump. One end of an outflowconduit is mechanically fitted to the pump outlet and the other end issurgically attached to the patient's aorta by anastomosis. Apercutaneous cable connects to the pump, exits the patient through anincision, and connects to the external control unit.

Various aspects of the implantable pump are similar to those shown anddescribed in U.S. Pat. Nos. 4,528,485; 4,857,781; 5,229,693; 5,588,812;5,708,346; 5,917,297; 6,100,618; 6,222,290; 6,249,067; 6,268,675;6,355,998; 6,351,048; 6,365,996; 6,522,093; 7,972,122; 8,686,674;8,770,945; U.S. Pub. No. 2014/0205467; 2012/0095281; and U.S. patentapplication Ser. No. 15/041,987, the entire contents of which patentsand publications are incorporated herein by this reference for allpurposes.

The exemplary system utilizes an implantable pump with contactlessbearings for supporting the impeller. Contactless bearings (i.e.,levitation) provide a number of potential benefits. Because they reducerotational friction, theoretically they improve motor efficiency andreduce the risk of introducing particulates into the fluid. In oneexample, the impeller employs upper and lower plates having magneticmaterials (the terminology of upper and lower being arbitrary since thepump can be operated in any orientation). A stationary magnetic fieldfrom the upper side of the pump housing attracts the upper plate and arotating magnetic field from the lower side of the pump housing attractsthe lower plate. The forces cooperate so that the impeller rotates at alevitated position within the pumping chamber. Features (not shown) mayalso be formed in the walls of the pumping chamber to produce ahydrodynamic bearing wherein forces from the circulating fluid also tendto center the impeller. Hydrodynamic pressure grooves adapted to providesuch a hydrodynamic bearing are shown in U.S. Pat. No. 7,470,246, issuedDec. 30, 2008, titled “Centrifugal Blood Pump Apparatus,” which isincorporated herein for all purposes by reference.

The exemplary impeller has an optimal location within the pumpingchamber with a predetermined spacing from the chamber walls on eachside. Maintaining a proper spacing limits the shear stress and the flowstasis of the pump. A high shear stress can cause hemolysis of the blood(i.e., damage to cells). Flow stasis can cause thrombosis (i.e., bloodclotting).

With continued reference to FIG. 1, a patient is shown in fragmentaryfront elevational view. Surgically implanted either into the patient'sabdominal cavity or pericardium 11 is the pumping unit 12 of aventricular assist device. An inflow conduit (on the hidden side of unit12) pierces the apex of the heart to convey blood from the patient'sleft ventricle into pumping unit 12. An outflow conduit 13 conveys bloodfrom pumping unit 12 to the patient's ascending aorta. A percutaneouspower cable 14 extends from pumping unit 12 outwardly of the patient'sbody via an incision to a compact control unit 15 worn by patient 10.Control unit 15 is powered by a main battery pack 16 and/or an externalAC power supply, and an internal backup battery. Control unit 15includes a commutator circuit for driving a motor within pumping unit12.

In various embodiments, the commutator circuit and/or variouselectronics may be on the implanted side of the system. For example,various electronics may be positioned on-board the pump or in a separatehermetically sealed housing. Among the potential advantages ofimplanting electronics is the ability to control the pump even whencommunication is lost with the control unit 15 outside the body.

FIG. 2 shows exemplary centrifugal pump unit 20 used in the system ofFIG. 1. The pump unit 20 includes an impeller 21 and a pump housinghaving upper and lower halves 22 a and 22 b. Impeller 21 is disposedwithin a pumping chamber 23 over a hub 24. Impeller 21 includes a firstplate or disc 25 and a second plate or disc 27 sandwiched over aplurality of vanes 26. Second disc 27 includes a plurality of embeddedmagnet segments 44 for interacting with a levitating magnetic fieldcreated by levitation magnet structure 34 disposed against housing 22 a.For achieving a small size, magnet structure 34 may comprise one or morepermanent magnet segments providing a symmetrical, static levitationmagnetic field around a 3600 circumference. First disc 25 also containsembedded magnet segments 45 for magnetically coupling with a magneticfield from a stator assembly 35 disposed against housing 22 b. Housing22 a includes an inlet 28 for receiving blood from a patient's ventricleand distributing it to vanes 26. Impeller 21 is preferably circular andhas an outer circumferential edge 30. By rotatably driving impeller 21in a pumping direction 31, the blood received at an inner edge ofimpeller 21 is carried to outer circumferential 30 and enters a voluteregion 32 within pumping chamber 23 at an increased pressure. Thepressurized blood flows out from an outlet 33 formed by housing features33 a and 33 b. A flow-dividing guide wall 36 may be provided withinvolute region 32 to help stabilize the overall flow and the forcesacting on impeller 21.

FIG. 3 shows an exemplary pump 10 similar to the pump shown in FIG. 2.FIG. 3 shows impeller 21 located in a balanced position. The balancedposition sometimes refers to the position the impeller naturallystabilizes or finds equilibrium during operation. In the exemplaryembodiment, the balanced position is at or near the center of the pumpchamber. In the balanced position, the forces acting on the impeller aregenerally balanced to stabilize the impeller. As one will understandfrom the description above that the hydrodynamic forces on the impellerwill change as the rotational speed of the impeller changes. In turn,the magnetic attractive forces on the impeller will change as theimpeller moves closer to or away from the magnet structure 34 and statorassembly 35. Accordingly, the impeller generally finds a new balancedposition as the rotational speed changes.

As will be described below, however, aspects of the invention aredirected to moving the impeller or changing the balanced position foreach given rotational speed. For example, the impeller position controlmechanisms to be described facilitate moving the impeller axially (up ordown) without changing the rotational speed and all other. This has theeffect of enabling movement of the impeller independent of rotor speed.An advantage of this technique is that rotor speed can be determined innormal course (e.g. by a physician based on the patient's physiologicalneeds) without concern for changing the impeller position. Conversely,the impeller position can be changed without affecting pumpingthroughput.

FIG. 3 shows impeller 21 located at or near a centered position whereindisc 27 is spaced from housing 22A by a gap 42 and impeller disc 25 isspaced from housing 22B by a gap 43. In the exemplary embodiment, thecenter position is chosen as the balanced or balanced position to ensuresubstantially uniform flow through gaps 42 and 43. During pumpoperation, the balanced position is maintained by the interaction of (a)attractive magnetic forces between permanent magnets 40 and 41 inlevitation magnet structure 34 with imbedded magnetic material 44 withinimpeller disc 27, (b) attractive magnetic forces between stator assembly35 and embedded magnet material 45 in impeller disc 25, and (c)hydrodynamic bearing forces exerted by the circulating fluid which maybe increased by forming hydrodynamic pressure grooves in housing 22 (notshown). By using permanent magnets in structure 34 a compact shape isrealized and potential failures associated with the complexities ofimplementing active levitation magnet control are avoided. To properlybalance impeller 21 at the centered position, however, and because otherforces acting on impeller 21 are not constant, an active positioningcontrol may be desired. In particular, the hydrodynamic forces acting onimpeller 21 vary according to the rotational speed of impeller 21.Furthermore, the attractive force applied to impeller 21 by statorassembly 35 depends on the magnitude of the magnetic field and the angleby which the magnetic field leads the impellers magnetic field position.

The structures and methods for controlling the motor will now bedescribed with references to FIGS. 4 to 6.

FIG. 4 shows the main structure of an exemplary centrifugal pump 50similar to that shown in FIG. 3. It is understood that other pumpconfigurations may be employed, including various combinations ofpermanent magnets, motor stator windings, and hydrodynamic bearings. Inthe exemplary embodiment, the rotor is formed as an impeller and drivenby a motor. The impeller is also levitated by the combined forceFa_(aΣ), which can be expressed as the following equation:

F _(aΣ) =F _(hdb) +F _(pm) +F _(em)

Where,

F_(aΣ) is the combined force to levitate the impeller

F_(hdb) is the combination of hydrodynamic forces from the inlet sidebearing, the motor side bearing, or both

F_(pm) is the combination of permanent magnet attraction forces

F_(em) is the magnetic attraction force generated from the motor

When the impeller is stabilized, F_(aΣ) should be equal to zero. UsuallyF_(em) can be controlled through the electronic system to adjust theimpeller position since all the others are the fixed configurations asthe passive mode. Therefore, the basic design concept of this inventionis to apply the motor vector control (FOC) to control the force F_(em)so that the impeller position can be adjusted while rotating only usingone set of motor coil and drive system. In such way, there is noadditional cost in the pump structure.

FIGS. 5 to 6 illustrate a method for controlling voltages applied to astator in order to provide a desired rotation for a permanent magnetrotor (e.g. the impeller) 52 is a field oriented control (FOC)algorithm, which is also known as vector control. It is known in FOCthat the stator magnetic field should generally lead the impellerposition by 90° for maximum torque efficiency. The magnitude of theattractive force on the impeller is proportional to the magnitude of thephase currents in the stator. Phase current is adjusted by the FOCalgorithm according to torque demands for the pump. Since thecommutation angle is typically fixed at 90°, the resulting attractiveforce varies according to torque output from the pump. The exemplarytechnique varies the I_(d) current which creates magnetic fluxresponsible for attracting the impeller. This provides a convenient andaccurate mechanism to create a controlled impeller attractive force.

At any particular combination of the (1) magnitude of the phase currentand (2) the speed of the impeller, modifying the commutation angle forgenerating the phase voltages can change the attractive force generatedby the stator thereby affecting the impeller balance. In turn, theimpeller moves until it settles at a new balanced position where thehydrodynamic forces and magnetic forces are balanced.

FIGS. 5, 6, 7 a, and 7 b illustrate an exemplary system in accordancewith aspects of the invention. Based on the principle of motor vectorcontrol, the torque current that is usually called quadrature current(I_(q) current) and stator coil flux current that is called directcurrent (I_(d) current) can be decoupled and controlled independently.The quadrature current I_(q) current is used to control the impellerrotational speed. The direct I_(d) current controls the magnetic flux ofelectromagnetic coils.

In accordance with the invention, I_(d) current is utilized to controlthe impeller position by enhancing or weakening the magnetic fluxbetween impeller (rotor) and motor stator coils to adjust the attractionforce F_(em). This in turn changes the impeller position (shown in FIG.6).

In one embodiment, the impeller position control technique isimplemented as an open loop control without impeller position sensors.In one embodiment, impeller position control technique is implemented asa closed loop control with impeller position sensors.

In order to ensure proper positioning, active monitoring and control ofthe impeller position has been employed in the exemplary embodiment byadjusting the stationary magnetic field. However, position sensors andan adjustable magnetic source occupy a significant amount of space andadd to the complexity of a system. Accordingly, the use of sensors maydepend on the design requirements. Suitable sensors may include, but arenot limited to, Hall-effect sensors, variable reluctance sensors, andaccelerometers.

In one embodiment using the open loop control, the impeller iscontrolled by periodically alternating the position from one side toanother (e.g. from inlet side to motor side) by modulating the I_(d)current as shown in FIG. 6. In this manner, the side gaps (Gap 1 and Gap2) as shown in FIGS. 7a and 7b can be increased or decreased.

The position control technique can be implemented into the hardwareand/or software of the system. Referring to FIG. 5, by example, thecontroller may employ FOC to supply a multiphase voltage signal to thestator assembly 53. The exemplary stator assembly is a three-phasestator. Individual phases a, b, and c and currents I_(a), I_(b), andI_(c) may be driven by an H-bridge inverter functioning as a commutationcircuit driven by a pulse width modulator (PWM) circuit. An optionalcurrent sensing circuit associated with the inverter measureinstantaneous phase current in at least two phases providing currentsignals designated I_(a) and I_(b). A current calculating block receivesthe two measured currents and calculates a current I_(c) correspondingto the third phase. The measured currents are input to Vector Control(FOC) block 54 and to a current observer block (not shown) whichestimates the position and speed of the impeller. The impeller positionand speed are input to the FOC block from speed control block 55 andposition control block 56. A target speed or revolutions per minute(rpm) for operating the pump is provided by a conventional physiologicalmonitor to FOC block 54. The target rpm may be set by a medicalcaregiver or determined according to an algorithm based on variouspatient parameters such heart beat, physiological needs, suctiondetection, and the like. FOC block 54 and drive electronics 57 generatecommanded voltage output values Va, Vb, and Vc. The Va, Vb, and Vccommands may also be coupled to the observer block for use in detectingspeed and position.

The exemplary system differs from conventional configurations inasmuchas the FOC block and electronics are configured to alter the fieldoriented control algorithm so that a direct current (Id) can be variedindependently and generate a desired attractive force.

In one embodiment, the invention proceeds according to a method as shownin FIG. 8 which highlights a portion of the impeller position controlwith the field oriented control algorithm. Thus, in step 65 the phasecurrents are measured. Based on the measured phase currents, the currentspeed and rotor angle of the impeller are estimated in step 66. Based onthe measured rotor angle in step 66, the phase currents are transformedinto a two-axis coordinate system to generate quadrature current (I_(q)current) and direct current (I_(d)) values in a rotating reference framein step 67. Quadrature current is used to control the torque to rotatethe impeller and direct current is used to control the attraction forcebetween rotor and stator to control the impeller position. In step 68,the next quadrature voltage is determined by the quadrature currenterror between the quadrature current transformed from step 67 and therequired current for impeller rotation. In step 69, the next directvoltage is determined by the direct current error between the directcurrent transformed from step 67 and the required current for theattraction force alternation to control the impeller position. In step70, the quadrature and direct voltages are transformed back to thestationary reference frame in order to provide the multiphase voltagecommands which are output to the PWM circuit.

FIG. 9a is a flowchart showing another method of operating a rotarymachine in accordance with the invention. The method includes operatingthe pump to rotate the impeller by applying a rotating magnetic field instep S10. During operation the impeller is levitated and positioned at abalanced position (P₁) by a balancing of forces. As described above, inan exemplary embodiment the impeller is levitated by the combination ofhydrodynamic forces F₁ and other bearing forces F₂ (e.g. statorattractive force, passive magnetic forces, and/or bulk forces likegravity) in steps S11 and S12. Next, at least one of the forces, F₂, ismodified to place the impeller out of balance in step S14. The impellermoves to a new position, P₂, where the forces are once again balanced instep S15.

As described above, the impeller is moved from the first balancedposition (P₁) to the second balanced position (P₂) by applying anattractive force or modifying (increasing or decreasing) an existingattractive force on the impeller. In various embodiments, the attractiveforce modulation is substantially continuously applied to hold theimpeller in the second balanced position. In various embodiments, theattractive force modulation is applied periodically (e.g. as pulses) tohold the impeller in the second balanced position. In variousembodiments, the attractive force modulation is applied as a singlepulse to move the impeller in the second balanced position. The secondbalanced position can be configured so the impeller remains in thesecond balanced position in a stable manner even when the attractiveforce is removed.

FIG. 9b illustrates an exemplary method for moving the impeller betweentwo balanced positions in accordance with aspects of the invention. Inthis example, a permanent magnet arrangement is contemplated whereby thetwo balanced positions are naturally a product of the permanent magnetarrangement, so that the above-mentioned attractive force modulation maynot necessarily be needed or required to hold the impeller in aparticular balanced position. Rather, a single (or series) pulse may beapplied to “push” the impeller from one stable position to another. Forexample, as shown in FIG. 9b , the impeller is initially, at a time t1,at a stable balanced position A. At a time t2 a pulse of duration(t3-t2) is applied to push the impeller to a new, stable balancedposition B. In practice, the impeller may be pushed between stablebalanced position A and stable balanced position B in a manner as neededor desired, such as shown in FIG. 9b . Advantageously, such animplementation may save energy and improve pump efficiency. In oneembodiment, one of the two stable positions may be relatively close tothe inlet of the above mentioned centrifugal pump unit, and anotherrelatively close to the motor.

Turning to FIG. 10, in one embodiment, the impeller position controltechnique is used to facilitate start-up of the pump. In step S20, theexemplary pump is configured so the impeller rests against the inletside (top of the housing) when the impeller is not rotating. In atypical pump with hydrodynamic forces alone, or in combination withmagnetic forces, the impeller is levitated away from the wall as itrotates. The blood entrained in the gap between the impeller and thehouse creates hydrodynamic pressure; however, the impeller must berotating at a sufficient speed to create the hydrodynamic pressure.Until the minimum speed is met, the impeller rubs against the housingwall. In the exemplary pump, by contrast, the impeller is pulled awayfrom the wall prior to, or just after, rotation begins therebyeliminating the deleterious effects of friction. The impeller is pulledaway from the wall by applying a force, F, as described above in stepS21. For example, the commutation angle may be modified to exert anattractive force. Referring to FIG. 7b , by example, the pump can beconfigured so the impeller rests at the inlet side. By applying anattractive force to the motor side the impeller moves down from the topwall. In step S22, the regular start-up sequence is initiated after theimpeller is removed from the wall.

FIG. 11 shows a rotary machine in accordance with another embodimentmaking use of electromagnets. Pump 100 in FIG. 11 is similar in variousrespects to pump 10 in FIG. 3. In the exemplary embodiment, however,pump 100 includes an active electromagnetic (EM) system 1010. The EMforce generated by electromagnets is used primarily or adjunctively tomove the impeller. Exemplary electromagnets 101 comprise iron cores andwindings. The EM force is modified in a conventional manner by changingthe current applied to the windings. The application of the EM forcecauses the impeller to move to position P_(E2). One will appreciate thatthe EM force can overpower hydrodynamic and passive magnetic forcespresent in the system. Accordingly, the EM structure must be dimensionedand configured to apply a relatively balanced force. An advantage ofusing electromagnets over the existing stator assembly is that there isrelatively greater positional control over the impeller. By contrast, asdescribed above, the phase currents typically cannot be used as theprimary variable to adjust the axial attractive force on the impeller. Adisadvantage of this embodiment is the need to provide an entirelyseparate EM system. This may not be an issue with large industrialrotary machines, but many types of motors have restrictive form factors.For example, implanted pumps must be relatively small in order toaddress a wider patient population.

FIGS. 12 and 13 illustrate another implantable pump in accordance withthe invention. Pump 200 is similar in various respects to pumps 10 and100 described above except pump 200 is an axial flow pump. Blood flowsfrom in through inlet 201 and out through outlet 202 in a generallylinear, axial direction. Pump 200 includes an impeller 210 having bladesfor moving blood through the pump housing and imparting kinetic energyin the fluid.

Impeller 210 is fixed within the housing by ball-and-cup bearings 212and 214. The ball-and-cup bearings are closely toleranced and generallyfix the impeller in a specific position. However, the exemplary bearingsare lubricated and washed by the blood flow around the impeller.Accordingly, there is some fluid between the ball and cup surfaces.

Torque is applied to the impeller by a stator assembly 205. The statorassembly 205 includes windings and is driven using a FOC algorithm in asimilar manner to the stator assemblies described above. In practice,the impeller position is adjusted proceeding according to the methodshown in FIG. 9. Using the FOC technique described above the impeller isrotated in the pump housing. At a desired time the I_(d) current ismodulated to adjust the attractive force on the impeller in the axialdirection. As long as the attractive force is sufficient to squeezeblood out from a respective bearing gap, the impeller will move axiallytowards inlet 201 or outlet 202. The bearing gaps of pump 200 arerelatively small compared to Gap 1 and Gap 2 of pump 50 in FIG. 4.However, even relatively small impeller movement may be beneficial toenable control of the bearing gaps.

FIG. 14 illustrates another pump 300 similar to pump 200. Pump 300includes an impeller fixed between two mechanical bearings 212 and 214.Pump 300 is slightly different than pump 200 because the outlet extendsat an angle from the inlet. Pump 300 is configured in a relativelycompact design compared to pump 200 including a relatively smallerstator assembly; however, the same general principles can be applied tocontrol the motor and adjust the impeller position.

FIG. 15 is a cross-sectional view of another pump 400 similar to pumps200 and 300, except pump 400 is an axial flow pump with non-contactbearings. Pump 400 includes a pump housing having an inlet 401 andoutlet 402. An impeller 411 is positioned within a pump chamber forimparting flow to the blood fluid within the housing. The impeller isentirely formed of a magnetic material which is driven by interactionwith a stator assembly 405.

Impeller 411 is stabilized in the pump chamber by a combination ofhydrodynamic and passive magnetic forces. Impeller 411, which is amagnetic material, interacts with the magnetic material in statorassembly 405 to provide an axial centering force. A pump ring 452 with achamfer surface is positioned at the leading end of the impeller tocreate hydrodynamic stabilization forces in the axial direction (left toright) and radial direction (up and down on page). A permanent magnetring 450 is provided at the trailing edge of the impeller is orientedwith a north pole facing a north pole of the impeller. This arrangementcreates an axial bias force to push the impeller against the pump ring452. The magnet ring 450 also provides a radially centering force.Finally, the impeller includes deep hydrodynamic grooves to generate ahydrodynamic pressure force against the inner walls of the pump chamberfor radial stabilization.

In operation, the impeller remains stable in the axial and radialdirections. There may be some axial movement as the rotational speed ofthe impeller changes or as a result of other forces (e.g. the nativepulse), but generally the impeller remains centered below the statorassembly.

Using the FOC control technique described above, the attractive force ofthe stator assembly 405 on impeller 411 can be modified. In oneembodiment, pump 400 is configured so impeller is eccentric whencentered below the stator assembly 405. In this example, increasing theattractive force amounts to an increase in the axial stiffness to resistaxial movement. In one embodiment, the attractive force is modified toactually move impeller 411 axially. For example, the impeller can bemoved closer to pump ring 452 to squeeze blood out of the gap betweenimpeller 411 and a surface of ring 452. The impeller may also be movedaway from ring 452 to increase the blood gap therebetween.

In this manner, the impeller position control technique adds an elementof active position control otherwise not possible with the passivebearing configuration of pump 400.

Although aspects of the invention have been described in connection withblood pumps, one will appreciate from the description herein that theinvention can be applied equally to other types of rotary machines suchas washing machines, manufacturing machines, computer drives, and more.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical application,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the Claims appended hereto and theirequivalents.

What is claimed is:
 1. A blood pump, comprising: a rotor disposed withina housing, wherein the housing is configured to cause a first axialforce on the rotor; a stator for: applying a rotating magnetic field tothe rotor; and applying a second axial force on the rotor; a controllerfor modifying phase voltages provided to the stator to modify the secondaxial force such that the rotor is moved from a first balanced positionto a second balanced position, wherein the second balanced position isdifferent than the first balanced position.
 2. The blood pump of claim1, wherein the housing being configured to cause the first axial forceon the rotor comprises: the housing comprising a first permanent magnetconfigured to repulse a second permanent magnet disposed in the rotor.3. The blood pump of claim 1, wherein the housing being configured tocause the first axial force on the rotor comprises: the housing defininghydrodynamic pressure grooves facing the rotor.
 4. The blood pump ofclaim 1, wherein the housing being configured to cause the first axialforce on the rotor comprises: the housing being configured to circulatefluid about the rotor.
 5. The blood pump of claim 4, wherein: the rotordefines hydrodynamic pressure grooves.
 6. The blood pump of claim 1,wherein the controller is further for: determining an axial position ofthe rotor; and modifying the second axial force based on the axialposition of the rotor.
 7. The blood pump of claim 1, wherein modifyingphase voltages comprises: continuously modifying the phase voltages tomove the rotor from the first balanced position to the second balancedposition and to hold the rotor in the second balanced position.
 8. Theblood pump of claim 1, wherein modifying phase voltages comprises:periodically modifying the phase voltages to move the rotor from thefirst balanced position to the second balanced position and to hold therotor in the second balanced position.
 9. The blood pump of claim 1,wherein modifying phase voltages comprises: modifying the phase voltagesin a single pulse to move the rotor from the first balanced position tothe second balanced position.
 10. The blood pump of claim 1, wherein:after the rotor has moved to the second balanced position, modifying thephase voltages is not required to maintain the stator in the secondbalanced position.
 11. A non-transitory machine readable medium havinginstructions stored thereon, wherein the instructions, when executed,cause at least one processor to perform operations comprising:determining an axial position of a rotor within a blood pump comprisinga housing configured to cause a first axial force on the rotor; causinga stator of the blood pump to apply a rotating magnetic field to therotor; causing the stator of the blood pump to apply a second axialforce on the rotor; and modifying phase voltages provided to the statorto modify the second axial force such that the rotor is moved from afirst balanced position to a second balanced position, wherein thesecond balanced position is different than the first balanced position.12. The non-transitory machine readable medium of claim 11, whereindetermining the axial positon of the rotor comprises: determining atleast one phase current; and determining the axial position of the rotorbased on the at least one phase current.
 13. The non-transitory machinereadable medium of claim 11, wherein determining the axial positon ofthe rotor comprises: receiving a signal from a Hall sensor; anddetermining the axial position of the rotor based on the signal.
 14. Thenon-transitory machine readable medium of claim 11, wherein determiningthe axial positon of the rotor comprises: receiving a signal from anoptical sensor; and determining the axial position of the rotor based onthe signal.
 15. The non-transitory machine readable medium of claim 11,wherein modifying phase voltages comprises: varying a commutation angleprovided by a commutator circuit to the stator.
 16. The non-transitorymachine readable medium of claim 11, wherein causing the stator of theblood pump to apply the rotating magnetic field to the rotor comprises:using a field oriented control algorithm.
 17. The non-transitory machinereadable medium of claim 11, wherein modifying phase voltages providedto the stator comprises: continuously modifying the phase voltages tomove the rotor from the first balanced position to the second balancedposition and to hold the rotor in the second balanced position.
 18. Thenon-transitory machine readable medium of claim 11, wherein modifyingphase voltages provided to the stator comprises: periodically modifyingthe phase voltages to move the rotor from the first balanced position tothe second balanced position and to hold the rotor in the secondbalanced position.
 19. The non-transitory machine readable medium ofclaim 11, wherein modifying phase voltages provided to the statorcomprises: modifying the phase voltages in a single pulse to move therotor from the first balanced position to the second balanced position.20. The non-transitory machine readable medium of claim 11, wherein:after the rotor has moved to the second balanced position, modifying thephase voltages is not required to maintain the stator in the secondbalanced position.